Reduction of artifacts in spectral computed tomography image data

Abstract

A method comprises: receiving computed tomography measurement data with sets of measurement values, wherein a set corresponds to a detector element readout of the measurement data including measurement values from at least two different x-ray spectra; determining an error estimate per set of measurement values; defining a replacement reference value per set of measurement values; applying a function that maps the measurement values to corrected values, the corrected values being between the mapped measurement value and the replacement reference value of the respective set, such that for larger errors the corrected value is closer to or at the replacement reference value and for smaller errors the corrected value is closer to or at the measurement value; and reconstructing computed tomography image data based on the corrected values.

What is claimed is: 
     
         1 . A computer-implemented method for reduction of artifacts in spectral computed tomography image data, the computer-implemented method comprising:
 receiving computed tomography measurement data with sets of measurement values, wherein each set of measurement values corresponds to a respective detector element readout and includes measurement values for at least two different x-ray spectra;   at least one of determining or receiving an error estimate for each set of measurement values;   defining a replacement reference value for each set of measurement values;   applying a function that maps measurement values to corrected values, wherein
 for each respective measurement value, a respective corrected value is between the respective measurement value and the replacement reference value for the set of measurement values including the respective measurement value, and 
 a distance of the respective corrected value to each respective measurement value depends on the error estimate such that for larger errors the respective corrected value is closer to or at the replacement reference value for the set of measurement values including the respective measurement value, and for smaller errors the respective corrected value is closer to or at the respective measurement value; and 
   reconstructing computed tomography image data based on the corrected values.   
     
     
         2 . The computer-implemented method according to  claim 1 , wherein the replacement reference value for the set of measurement values including the respective measurement value is in an order of magnitude of the measurement values of the set of measurement values including the respective measurement value. 
     
     
         3 . The computer-implemented method according to  claim 1 , wherein the replacement reference value for the set of measurement values including the respective measurement value is defined such that the replacement reference value is a linear combination of the measurement values of the set of measurement values including the respective measurement value. 
     
     
         4 . The computer-implemented method according to  claim 3 , wherein at least one of coefficients of the linear combination are the same within each mapped set, or a sum of the coefficients of the linear combination is 1 or substantially 1. 
     
     
         5 . The computer-implemented method according to  claim 1 , wherein the replacement reference value for the set of measurement values including the respective measurement value is defined to be one of the measurement values in the set of measurement values including the respective measurement value. 
     
     
         6 . The computer-implemented method according to  claim 1 , wherein the error estimate is based on determining an error of data that is erroneous due to scattered radiation. 
     
     
         7 . The computer-implemented method according to  claim 6 , wherein the error estimate is determined based on a ratio of an estimated scatter signal to a total signal strength of the measurement values of the set of measurement values including the respective measurement value. 
     
     
         8 . The computer-implemented method according to  claim 1 , wherein the error estimate is determined based on data errors in a low signal domain, such that a high error is assumed for low detector signals. 
     
     
         9 . The computer-implemented method according to  claim 1 , wherein the function that maps the measurement values to the corrected values is a monotonic function. 
     
     
         10 . The computer-implemented method according to  claim 1 , wherein the mapping is defined according to at least two different error ranges of the error estimate, such that for each error range a different partial mapping function is applied. 
     
     
         11 . The computer-implemented method according to  claim 1 , wherein
 the error estimate includes at least three error ranges according to which the mapping is applied, and   the error ranges include
 a first error range of low errors for which the respective corrected value is set to the respective measurement value, 
 a second error range of medium errors for which the measurement values are mapped such that the respective corrected value is between the respective measurement value and the replacement reference value for the set of measurement values including the respective measurement value, and 
 a third error range of high errors for which the respective corrected value is set to be the replacement reference value for the set of measurement values including the respective measurement value. 
   
     
     
         12 . A method for providing spectral computed tomography image data, the method comprising:
 acquiring measurement data by performing a spectral computed tomography scan; and   performing the computer-implemented method according to  claim 1 .   
     
     
         13 . A non-transitory computer-readable storage medium storing computer-executable instructions that, when executed by a computer, cause the computer to carry out the computer-implemented method of  claim 1 . 
     
     
         14 . A computed tomography system comprising a processing unit configured to carry out the computer-implemented method according to  claim 1 . 
     
     
         15 . The computer-implemented method according to  claim 7 , wherein the ratio is a maximum ratio between the estimated scatter signal and the total signal strength. 
     
     
         16 . The computer-implemented method according to  claim 2 , wherein the replacement reference value for the set of measurement values including the respective measurement value is defined such that the replacement reference value is a linear combination of the measurement values of the set of measurement values including the respective measurement value. 
     
     
         17 . The computer-implemented method according to  claim 4 , wherein the replacement reference value for the set of measurement values including the respective measurement value is defined to be one of the measurement values of the set of measurement values including the respective measurement value. 
     
     
         18 . The computer-implemented method according to  claim 7 , wherein the mapping is defined according to at least two different error ranges of the error estimate, such that for each error range a different partial mapping function is applied. 
     
     
         19 . The computer-implemented method according to  claim 7 , wherein
 the error estimate includes at least three error ranges according to which the mapping is applied, and   the error ranges include
 a first error range of low errors for which the respective corrected value is set to the respective measurement value, 
 a second error range of medium errors for which the measurement values are mapped such that the respective corrected value is between the respective measurement value and the replacement reference value for the set of measurement values including the respective measurement value, and 
 a third error range of high errors for which the respective corrected value is set to be the replacement reference value for the set of measurement values including the respective measurement value. 
   
     
     
         20 . A computed-tomography system for reduction of artifacts in spectral computed tomography image data, the computed-tomography system comprising:
 a memory storing computer-readable instructions; and   at least one processor configured to execute the computer-readable instructions to cause the computed-tomography system to
 receive computed tomography measurement data with sets of measurement values, wherein each set of measurement values corresponds to a respective detector element readout and includes measurement values for at least two different x-ray spectra, 
 at least one of determine or receive an error estimate for each set of measurement values, 
 define a replacement reference value for each set of measurement values, 
 apply a function that maps measurement values to corrected values, wherein
 for each respective measurement value, a respective corrected value is between the respective measurement value and the replacement reference value for the set of measurement values including the respective measurement value, and 
 a distance of the respective corrected value to each respective measurement value depends on the error estimate such that for larger errors the respective corrected value is closer to or at the replacement reference value for the set of measurement values including the respective measurement value, and for smaller errors the respective corrected value is closer to or at the respective measurement value, and 
 
 reconstruct computed tomography image data based on the corrected values.